1. Field of the Invention
The present invention relates to an injectible two-staged rotary compressor and a heat pump system.
2. Description of the Related Art
The gas injection cycle is advantageous in that it increases the amount of refrigerant circulated through a heat radiator, and improves a heat-radiating capacity (heater capacity or water heater capacity). These advantages are achieved by having a structure in which a compressor sucks in additional refrigerant also during a compression process. Especially in cold regions, the amount of the circulated refrigerant decreases, because a base gas sucked into the compressor is diluted because of cold; therefore, it is effective to increase the amount of circuited refrigerant by an injection. Even if the injection is performed during the compression process, the amount of the refrigerant circulating through an evaporator stays the same, because the amount of the circulated refrigerant is determined by a basic displacement capacity and a rotation frequency of the compressor. However, it is possible to improve evaporating capacity (cooler capacity) too, by liquefying the refrigerant in a gas-liquid separator, or providing additional overcooling in an internal heat exchanger at an entry point to the evaporator.
In such a gas injection cycle, it is known that the compressor efficiency can be improved by mixing a small amount of liquefied refrigerant to the refrigerant to be injected to the compressor, partly because the liquefied refrigerant has a cooling effect on the compressor (for an example, see Japanese Patent Application Laid-Open No. 2004-85019). In addition, to maintain the reliability of a compressor, the compressor must be limited in operating pressure ratio and rotation frequency. This is because the higher the operating pressure ratio and the rotation frequency the compressor become, the more the compressor is heated up. Because of the cooling effect described above, these limitations can also be advantageously alleviated.
However, in the conventional gas injection cycle, the reliability decreases if too much liquefied refrigerant is mixed into the injected refrigerant. Because, too much of liquefied refrigerant reduces the viscosity of the lubricants, causing defective lubrication or defective sealing, and increase in bearing loads with still more liquefied refrigerant being mixed (for an example, see Japanese Patent Application Laid-Open No. 11-132575).
In other words, an appropriate amount of the liquefied refrigerant must be mixed to the refrigerant before the refrigerant is sucked into the compressor. The conventional documents teach methods of mixing the liquefied refrigerant and the injected refrigerant in an appropriate ratio, i.e., controlling a variable expansion valve or a flow-rate controlling valve in the gas injection cycle.
There has been a need to further improve the efficiency of the compressor.